For fans of Benicio Del Toro or of director Michael Mann, a visit to the cinema this weekend will have proved a tantalising experience. There, among the trailers, they might have found a sneak preview of Lucky Star, a peppy-looking thriller made with the same stylised flourish Mann brought to Heat, The Insider and Ali, starring the professionally addled Del Toro as a master gambler cleaning up at both casino and stock market. His judgment, we learn, is faultless, his wagers blessed with luck remarkable enough to attract the suspicions of various murky government agents, who duly hound him into leaving town in a gleaming silver convertible. But where is he going? What destiny awaits him? Fade to black. Coming Soon.

Except, in all probability, it won't be, because, while Lucky Star may have the frantic, elliptical form of a trailer, and while it may boast a movie-star male lead and Mann's sheeny (if uncredited) direction, there is no film to actually promote here. Rather, Lucky Star is an advert, its remit not to entertain but to persuade us of the virtues of the new Mercedes SL, the open-top sports car in which Del Toro is glimpsed absconding.

Despite such an apparent conceptual flaw, Mercedes' commercial is running first among the coming attractions, after more prosaic ads have left the screen. In print, the cod-journalistic flannel known as the advertorial is legally required to make an advert's purpose clear, but no such stricture exists at the cinema. Instead, the placement of a commercial within the programme is wholly at the discretion of that programme's distributors.

The placement of Mann's handiwork is a coup for advertising agency Campbell Doyle Dye - and a black day, perhaps, for those unnerved by the fondness of major corporations for blurring the lines between art and commerce, between novelty and expansionism.

Of course, big-name actors and film-makers dabbling in advertising is nothing new (David Lynch, for example, has spent his career hawking everything from Swiss cigarettes to Alka-Seltzer). But Mercedes' wool-over-the-eyes routine heralds a new era of borrowed credibility. Beyond appropriating the grammar of cinema or employing Hollywood's better-known talent, beyond the glitz-by-association of last year's BMW-funded The Hire (five short films produced by acclaimed directors revolving around the company's wares), Lucky Star accelerates us into a strange grey zone of marketing by subterfuge.

What Mann and Mercedes have done is turn the traditional device of product placement on its head. Now, rather than a movie discreetly (or otherwise) slipping a logo onto a set or costume, the product - in this case, Del Toro's shiny silver Merc - becomes the centrepiece, with characters and storylines called into existence as window dressing.

Not that Campbell Doyle Dye are going about things heavy-handedly. Far from it: the Mercedes branding of Lucky Star is subtle, verging on imperceptible. Those who watch the trailer and are eager to find out more will unearth not a high-octane thriller, but an invitation to their nearest Mercedes showroom.

Whether Mann's glacé camerawork will then be enough to sustain that interest once they notice the car's £92,000 price tag is moot. For Mercedes, it is evidently a risk worth taking. The prize, after all, is precious indeed: the neutering of our scepticism when confronted with advertising. Because, while making an ad in the form of a trailer could just about qualify as witty sleight of hand, actually trying to pass it off as a trailer is something more shady entirely.

Watching commercials, we come armed with a certain set of expectations - an evolved, fine-tuned mistrust. When we take in a movie (trailers included), we do so with those defences down, and necessarily so. To enjoy a film (or play, or novel), it is vital we suspend our disbelief, accept and engage. Watching advertising, however elegantly cloaked, we have to brandish that disbelief like a crucifix. At this point Lucky Star begins to look less like a canny innovation and more a like stealth attack on potential audience resistance.

All of which makes Mann's involvement puzzling. Certainly, his role as the creator of 1980s cop opera Miami Vice could betray a predilection for fast cars; more recently, the director has worked hard to position himself as a film-maker with no quiet social conscience, littering Ali with anti-establishment motifs and eviscerating the tobacco industry during The Insider. To the cynical eye, the step from lionising radical icons and corporate whistleblowers to promoting sports cars might seem just a touch inconsistent. (His fee for making the ad remains confidential.)

But one explanation may be found in the chance that Lucky Star might become a real movie after all. Mann has apparently optioned the rights with a view to development as a feature. Story ideas (even those as scanty as this) are gold - and, in an age where cinematic source material is routinely mined from computer games and bad 1970s TV shows, perhaps we shouldn't be too surprised by a film being adapted from its own trailer, which is actually an advert in the first place. The results may not be art; but then, art rarely bought anyone a new Mercedes-Benz.